If you're indoors, you're inside a house or other building. On the hottest, most sweltering summer days, you might decide to stay indoors where it's cool.
During a typical school day, elementary school students spend most of their time indoors, reading, writing, doing math, singing songs, and painting pictures. When they leave the indoors and go outside for recess, you can say they're outdoors. Indoors, first used around 1800 (sometimes attributed to George Washington), comes from indoor, a shortened form of within door.